1. Arnaud, M. (1994) these proceedings, p. 197.
2. Arnaud, M., Rothenflug, R., Boulade, V., Vigroux, L., and Vangioni-Flam, E. (1992) Astr. Astrophys. 254, 49.
3. Barcons, X., Fabian, A.C., and Rees, M. (1991) Nature 350, 685.
4. Binggeli, B., Tammann, G.A., and Sandage, A. (1987) Astr. J. 94, 251.
5. Binggeli, B., Popescu, C.C., and Tammann, G.A. 1993, Astr. Astrophys. Suppl. 98, 275.